Thermoprocessable copolymers of TFE

ABSTRACT

A thermoprocessable copolymer of TFE essentially formed by:  
     (a) from 4.5 to 8.5% by weight of perfluoromethylvinylether (PMVE);  
     (b) from 0.3 to 1.6% by weight of a perfluorodioxole of formula:  
                 
 
     wherein: Y=F, OR f , R f  wherein R f  is a perfluoroalkyl having from 1 to 5 carbon atoms;  
     X 1  and X 2 , equal to or diferent from each other, are —F or —CF 3 ;  
     (c) TFE, forming the remaining part to 100%.

[0001] The present invention relates to thermoprocessable perfluorinatedpolymers for the preparation of manufactured articles used in thesemicon industry, which show very low values of extractable F⁻, both onperfluoropolymer pellet and the manufactured articles therefrom.

[0002] Specifically, the present invention relates to thermoprocessablecopolymers of tetrafluoroethylene (TFE) which show very low values ofextractable F⁻ combined with very good mechanical and elastomericproperties combine wherefore they result particularly suitable to beused in the semicon industry in the preparation of pipes, fittings andtanks for the storage and the transportation of chemical compounds andof ultrapure water. Besides, the thermoprocessable copolymers oftetrafluoroethylene (TFE) of the invention show very low values ofextractable Cl⁻, lower than 0.2 ppm by weight with respect to thepolymer weight, preferably lower than the analytical detection limits.

[0003] It is well known that in the semicon industry tanks and pipingsystems (pipes and fittings) are made of fluorinated polymer materialswhich during the use must release a minimum amount of F⁻, lower than orequal to 1 ppm, so as not to contaminate the transported fluids, thusavoiding the damaging of silicon-based wafers. In the paper “FluorideContamination from Fluoropolymers in Semiconductor Manufacture”published on “State Solid Technology” pages 65-68, July 1990, it isstated that for the semicon industry it would be desirable to obtainfinished manufactured articles which during the use release a minimumamount of F⁻, in particular lower than or equal to 1 ppm.

[0004] The manufactured articles for the semicon industry are generallyprepared with thermoprocessable copolymers of TFE andperfluoropropylvinylether (PPVE), belonging to the PFA class, whereinPPVE is about 3.5-4.5% by weight. Said copolymers are preferablyobtained by a polymerization process in aqueous emulsion which allows toobtain a high productivity and the formation of polymer structureshaving a high molecular weight, therefore characterized by goodmechanical properties and high ductility. However the TFE/PPVEcopolymers contain a certain amount of chain end groups of ionic type—CF₂COOH and of —COF type. The —COF end group, as a consequence ofhydrolysis reactions which occur during the polymerization itself, canbe transformed into ionic —CF₂COOH end group. In the processing, forexample for the preparation of pipes or fittings, said end groups candecompose producing hydrofluoric acid HF. The formed hydrofluoric acidis released during the time from the manufactured article during the usein the semicon production plant, causing an unacceptable corrosion ofsilicon-based wafers. To minimize the HF formation during the processingso that the finished manufactured article shows the minimum amount ofextractable F-, the prior art uses a fluorination process to transformthe end groups into stable perfluorinated groups. See for example U.S.Pat. No. 4,743,658, wherein the TFE/PPVE copolymer is subjected tofluorination with elemental fluorine for reducing the amount of ionicend groups so that the fluorine extractable from the finishedmanufactured article is lower than 3 ppm by weight with respect to thepolymer. Said process requires an additional step and from theindustrial point of view it is not easily feasible since it useselemental fluorine, very aggressive agent which requires specialequipments for making the treatment safe and reliable.

[0005] Other processes for reducing the ionic end group number inperfluorinated copolymes are known. See for example U.S. Pat. No.5,093,409 wherein the TFE/PPVE copolymer under the form of latex istreated with amines at 160°-400° C. for a sufficient time to convert theionic —CF₂COOH end groups into —CF₂H groups. Also this post-treatmentfor stabilizing the fluoropolymer requires a specific unit and itresults therefore expensive from the economic point of view.

[0006] A class of thermoprocessable copolymers of TFE having very highchemical inertia and thermal stability is described in U.S. Pat. No.5,463,006, wherein terpolymers formed by TFE/PPVE/PMVE(perfluoromethylvinylether) are described. Tests carried out by theApplicant, see the comparative Examples, show that with theseterpolymers release values of F⁻ lower than 1 ppm are not obtained bothon the polymer (pellet) and on the finished manufactured article.Therefore also in this case it is necessary to carry out one of theabove post-treatments for reducing the values of extractable F⁻.

[0007] In conclusion, the thermoprocessable TFE copolymers of the priorart used for pipes in the semicon industry show a very good combinationof properties, such high flex life, good mechanical properties at highand low temperature and release values of F⁻ lower than 1 ppm. Howeverthe drawback of said products is that the combination of said propertiesis obtained with the proviso to subject them to a specific treatment offluorination or conversion of the end groups of the previously describedtype. All this requires a further processing step and an additionalspecific unit, making therefore difficult the process from theindustrial point of view and making it expensive from the economic pointof view.

[0008] Besides values of extractable F⁻ lower than or equal to 1 ppm,good mechanical properties both at low (23° C.) and high temperature(250° C.), good flex life, it would be desirable to carry out theprocessing of the manufactured articles, for example pipes, withoutcompromising the productivity thereof. At present in the preparation ofpipes having a low release of F⁻ for the semicon industry,perfluoropolymers post-treated as above indicated are absolutelynecessary and not high extrusion rates are used.

[0009] The need was therefore felt to have available in the semiconindustry a thermoprocessable fluoropolymer able to give finishedmanufactured articles having the following combination of proprties:

[0010] release values of F⁻ ions, both on the polymer pellet and on thefinished manufactured article, lower than 1 ppm by weight with respectto the polymer weight;

[0011] said release values of F⁻ ions lower than 1 ppm, in the case ofextruded manufactured articles (pipes), are obtainable also at highextrusion rate;

[0012] very low values of extractable Cl⁻, lower than 0.2 ppm by weightwith respect to the polymer weight, preferably lower than the analyticallimits;

[0013] flex life values higher than 20,000 combined with a MFI range(measured at 372° C. with a 5 Kg load) comprised between 1 and 5.

[0014] very good mechanical properties both at low temperature (23° C.)and at high temperature (250° C.).

[0015] The Applicant has surprisingly and unexpectedly found that it ispossible to obtain the combination of the above properties by using thespecific monomeric composition of thermoprocessable copolymers of TFE asdefined hereunder.

[0016] An object of the present invention is therefore athermoprocessable copolymer of TFE consisting essentially of:

[0017] (a) from 4.5 to 8.5% by weight of perfluoromethylvinylether(PMVE);

[0018] (b) from 0.3 to 1.6% by weight of a perfluorodioxole of formula:

[0019] wherein: Y=F, OR_(f), R_(f) wherein R_(f) is a perfluoroalkylhaving from 1 to 5 carbon atoms;

[0020] X₁ and X₂, equal to or diferent from each other, are —F or —CF₃;

[0021] (c) TFE, forming the remaining part to 100%;

[0022] said copolymer having:

[0023] a number of ionic end groups lower than 4×10⁻⁴ mol/Kg of polymer;

[0024] flex life values higher than 20,000 in a MFI range (measured at372° C. with a 5 Kg load) comprised between 1 and 5;

[0025] release values of F⁻ ions, both on the polymer in pellet and onthe finished manufactured article, lower than 1 ppm by weight withrespect to the polymer weight;

[0026] release values of F⁻ ions lower than 1 ppm on extrudedmanufactured articles (pipes), obtained both at low and high extrusionrate of the polymer.

[0027] The ionic end groups are for example of —COOH and —COF type.

[0028] Among the comonomers (b) the one wherein Y=F, X₁,X₂=CF₃ can forexample be mentioned; said compound isperfluoro-2,2-dimethyl-1,3-dioxole (PDD). See for example U.S. Pat. No.3,865,845.

[0029] Preferably in the present invention the compound of formula (I)wherein Y=OR where R_(f)=—CF₃; X₁,X₂=F is used as monomer (b). Saidcompound is named 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD).

[0030] The copolymers of the invention, besides showing very goodmechanical properties both at hot and cold as above mentioned, and verygood flex life, unexpectedly do not require any treatment offluorination or transformation of the end groups of the type of thoseabove described, for obtaining extractable values of F⁻ lower than 1 ppmon the finished manufactured articles. This result in the case ofextruded manufactured articles, for example pipes, is surprising sinceit is obtainable at various extrusion rates of the manufactured articleand do not penalize the productivity during the material processing (seethe Examples). In fact one can operate also at an extrusion rate doublethan those at present used in the manufactured article processing forthe semicon industry, without having a release of F⁻ ions higher than 1ppm. This is extremely important from the industrial point of view sincethe productivity is doubled.

[0031] To obtain the above combination of properties which makes thefinished manufactured articles suitable to be used in the semiconindustry, it has been found by the Applicant that it is necessary to usein polymerization comonomers (a) and (b) in the above defined amounts sothat the amount of ionic end groups is the above mentioned one. In fact,tests carried out by the Applicant (see the comparative Examples) showthat when the concentrations of perfluorodioxole orperfluoromethylvinylether are higher than the indicated limits, there isa worsening of the mechanical properties in hot conditions, inparticular the stress at break, measured at 250° C., becomes lower than5 MPa and the elongation at break (at 250° C.) becomes lower than 350%,which are the minimum acceptable values for a commercial fluoropolymerHYFLON^(R) MFA 620. When on the contrary the concentrations ofperfluoromethylvinylether or perfluorodioxole are lower than theindicated values, the finished polymer shows low flex life values andtherefore it does not result satisfactory in the applications of semiconindustry.

[0032] When the copolymers of the invention are used for obtaining pipesby extrusion, they preferably have a MFI in the range 1-5 measured at372° C. with a 5 kg load.

[0033] When the copolymers of the invention are used for obtaining bymoulding fittings, connections for pipings, etc., they preferably have aMFI comprised between 6 and 30 measured at 372° C. with a 5 kg load.

[0034] The copolymers of the invention can be obtained by polymerizingthe monomers by radical route both in aqueous and in organic medium. Thepolymerization in aqueous medium can be carried out in emulsion or inmicroemulsion in the presence of a radical inorganic initiator such asfor example the ammonium and/or potassium and/or sodium persulphate,optionally in combination with ferrous, cuprous or silver salts. Theinitiator feeding can be made in a continuous way or by a singleaddition at the starting of the polymerization. This latter method ispreferable since it reduces the polymerization times, the ionic endgroups in the polymer being equal. It has been found by the Applicantthat the used initiator amount must be low. For example by operating ata temperature of 75° C. with a pressure of 22 absolute bar, theinitiator amount, all fed at the starting of the polymerization, and inan amount lower than 0.03 grams of potassium persulphate for litre ofwater, allows to obtain the above indicated number of ionic end groups.

[0035] To obtain the results of the present invention it is preferablethat the perfluorodioxole monomer (b) is fed in a continuous way duringthe whole polymerization.

[0036] The synthesis temperature can be in the range 25°-120° C. Atemperature range 60°-95° C. is preferred when polymerization is carriedout in aqueous emulsion or microemulsion in the presence ofpersulphates. The polymerization can take place at pressures in therange 10-50 bar.

[0037] The polymerization in aqueous medium requires the presence of asurfactant, fluorinated surfactants such as perfluorooctanoate orammonium, potassium or sodium perfluorooctanoate, perfluorononanoate,perfluorodecanoate mixtures are particularly preferred. It isparticularly suitable to carry out the polymerization in aqueous phasein the presence of perfluoropolyethers as surfactants. Suchperfluoropolyethers can be added to the reaction medium under the formof a microemulsion, as described in U.S. Pat. No. 4,864,006.

[0038] For the control of the molecular weight of the inventionterpolymers, chain transfer agents such as hydrogen, methane, ethane,propane are used; which give end groups of hydrogenated type, suitabletherefore to the applications of the invention in the semicon industry.Chlorinated transfer agents are not used since it has been found thatthey produce amounts of extractable Cl⁻ dangerous for the semiconindustry.

[0039] The polymerization latex is coagulated with a coagulant such asfor example nitric acid; then the slurry washing and the subsequentdrying of the wet polymer take place. The powder is then pelletized in atwin-screw extruder equipped with at least two degassing zones.

[0040] The present invention will be better illustrated by the followingExamples, which have a merely indicative but not limitative purpose ofthe scope of the invention itself.

EXAMPLES

[0041] Characterization

[0042] The following measurements are carried out on the copolymergranules of the invention:

[0043] MFI at 5 Kg and 372° C. according to ASTM D-1238-52T method.

[0044] DSC for the measurement of the 2^(nd) melting temperatureaccording to the procedure described in U.S. Pat. No. 5,463,006 in thename of the Applicant.

[0045] Tensile properties (stress and elongation at break, yield stressand Young modulus) at room remperature and at T=250° C.

[0046] MIT Flex Life according to the procedure described in U.S. Pat.No. 5,463,006.

[0047] Determination of the chain end groups by IR analysis carried outon pellet by cold compression as described in the paper “End-groups inFluoropolymer” published by the review Journal of Fluorine Chemistry 95(1999), pages 71-84;

[0048] Determination of the monomeric composition by IR analysis;

[0049] Determination of the release of the F⁻ anions on pellets at 85°C. by ionic chromatography. The specimen preparation is carried out byusing KaPak®/Scotchpack vessels filled with deionized water at 18 MΩ andan amount equal to 5 grams of pellets. The vessels containing thepolymer specimen are closed and kept in a bath at 85° C. for 24 hours.Then the water contained in the vessels is analyzed by ionicchromatography, according to the following conditions:

[0050] column and precolumn AS4A+AG4A−Dionex;

[0051] eluent: Na₂B₄O, 3.0 millimolar, flow 1 cc/min;

[0052] Detection: suppressed conductivity;

[0053] the pellets have been extruded for obtaining pipes by using anextruder having a 45 mm diameter.

[0054] The extruder set-up has the following geometricalcharacteristics:

[0055] die diameter=53.58 mm; tip diameter=44.73 mm; pipe externaldiameter=12 mm; polymer thickness=1 mm; DDR=20 DRB=0.99. The followingtemperature profile has been used:

[0056] barrel temperature zone 1=325° C.; barrel temperature zone 2 and3=330° C.; barrel temperature zone 4 and collar=335° C.; necktemperature=325° C.; body temperature and die holder=330° C.; dietemperature=335° C.; melted temperature=342° C. Extrusion rates equal to0.7 and 1.8 meters/min are used in the Examples.

[0057] Also on the extruded pipes the released F⁻ analysis is carriedout by using the same procedures described for pellets, with thedifference that in the KaPak®/Scotchpack vessels a piece of pipe havinga weight equal to 5 grams is inserted.

Example 1

[0058] In a 22 l AISI 316 steel vertical autoclave, equipped withstirrer working at 400 rpm, after vacuum having been made, 13.9 litresof demineralized water, 0.5 g of2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD), and 160 g of anaqueous microemulsion of perfluoropolyether prepared according to theprocedures described in Example 1 of patent U.S. Pat. No. 4,864,006, areintroduced in sequence.

[0059] The autoclave is heated up to the reaction temperature equal to75° C.: 0.37 bar of ethane and 2.0 absolute bar ofperfluoromethylvinylether (PMVE) are then fed. A gaseous mixture havingthe following molar ratios: TFE/PMVE=34.97; PMVE/TTD=7.91;TFE/TTD=276.8, is fed by a compressor up to a pressure of 22 absolutebar.

[0060] The composition of the gaseous mixture present in the autoclavetop is analyzed by gaschromatography. Before the reaction starts, thegas phase results to be formed by the following molar percentages: 87.7%TFE, 11% PMVE, 0.3% TTD, 1% Ethane. Then by a metering pump 72 cm³ of apotassium persulphate 0.0103 molar solution are fed.

[0061] The polymerization pressure is maintained constant by feeding themonomeric mixture having the above defined ratios among the monomers andwhen 8,800 g of the aforesaid mixture have been fed, the reaction isstopped. The reactor is cooled to room temperature, the emulsion isdischarged and coagulated with HNO₃ (65%). Subsequently the polymer iswashed with H₂O and dried at about 220° C.

[0062] The copolymer composition by weight is equal to: PMVE  4.7% TTD 0.72% TFE 94.58%

[0063] Polymer properties, the number of ionic end groups (measured byIR), and the value of extractable F⁻ on the granule are reported inTable 1.

[0064] Mechanical properties both under cold (23° C.) and under heating(250° C.) and the flex-life values are reported in Table 2.

Example 2

[0065] In the same autoclave used in Example 1, after vacuum having beenmade, 13.9 litres of demineralized water, 0.5 g of TTD, and 160 g of theabove mentioned microemulsion, are in sequence introduced.

[0066] The autoclave is heated up to the reaction temperature equal to75° C.: 0.34 bar of ethane and 2.2 bar of PMVE are then introduced. Agaseous mixture having the following molar ratios: TFE/PMVE=32.22;PMVE/TTD=8.57; TFE/TTD=276.14 is fed by a compressor up to a pressure of22 absolute bar.

[0067] The composition of the gaseous mixture present in the autoclavetop is analyzed by gaschromatography. Before the reaction starts, thegas phase results to be formed by the following molar percentages:87.28% TFE, 11.5% PMVE, 0.3% TTD, 0.92% Ethane. Then by a metering pump72 cm³ of a potassium persulphate 0.0103 molar solution are fed.

[0068] The polymerization pressure is maintained constant by feeding themonomeric mixture having the above defined ratios among the monomers andwhen 8,800 g of the aforesaid mixture have been fed, the reaction isstopped. The reactor is cooled to room temperature, the emulsion isdischarged and coagulated with HNO₃ (65%) Subsequently the polymer iswashed with H₂O and dried at about 220° C.

[0069] The copolymer composition by weight is equal to: PMVE  5.1% TTD 0.72% TPE 94.18%

[0070] Polymer properties, the number of ionic end groups (measured byIR), and the value of extractable F⁻ on the granule are reported inTable 1.

[0071] Mechanical properties both under cold (23° C.) and under heating(250° C.) and the flex-life values are reported in Table 2.

Example 3

[0072] In the same autoclave used in Example 1, after vacuum having beenmade, 13.9 litres of demineralized water, 0.1 g of TTD, and 160 g of theabove mentioned microemulsion, are in sequence introduced.

[0073] The autoclave is heated up to the reaction temperature equal to75° C.: 0.36 bar of Ethane and 2.86 bar of PMVE are then introduced. Agaseous mixture having the following molar ratios: TFE/PMVE=27.51;PMVE/TTD=17.5; TFE/TTD=481.4 is fed by a compressor up to a pressure of22 absolute bar.

[0074] The composition of the gaseous mixture present in the autoclavetop is analyzed by gaschromatography. Before the reaction starts, thegas phase results to be formed by the following molar percentages:82.82% TFE, 16% PMVE, 0.18% TTD, 1% Ethane. Then by a metering pump 72cm³ of a potassium persulphate 0.0103 molar solution are fed.

[0075] The polymerization pressure is maintained constant by feeding themonomeric mixture having the above defined ratios among the monomers andwhen 8,800 g of the aforesaid mixture have been fed, the reaction isstopped. The reactor is cooled to room temperature, the emulsiondischarged and coagulated with HNO₃ (65%). Subsequently the polymer iswashed with H₂O and dried at about 220° C.

[0076] The copolymer composition by weight is equal to: PMVE  6.1% TTD 0.41% TFE 93.49%

[0077] Polymer properties, the number of ionic end groups (measured byIR), and the value of extractable F⁻ on the granule are reported inTable 1.

[0078] Mechanical properties both under cold (23° C.) and under heating(250° C.) and the flex-life values are reported in Table 2.

Example 4

[0079] One works with the same autoclave of Example 1. After vacuumhaving been made, 13.9 litres of demineralized water, 0.5 g of TTD, and160 g of the above mentioned microemulsion, are in sequence introduced.The autoclave is heated up to the reaction temperature equal to 75° C.:0.35 bar of Ethane and 2.86 absolute bar of PMVE are then introduced. Agaseous mixture having the following molar ratios: TFE/PMVE=27.47;PMVE/TTD=10.0; TFE/TTD=285.71 is fed by a compressor up to a pressure of22 absolute bar.

[0080] Before the reaction starts, the gas phase results to be formed bythe following molar percentages: 82.6% TFE, 16.05% PMVE, 0.35% TTD, 1%Ethane. Then by a metering pump 72 cm³ of a potassium persulphate 0.0103molar solution are fed.

[0081] The polymerization pressure is maintained constant by feeding themonomeric mixture having the above defined ratios among the monomers andwhen 8,800 g of the aforesaid mixture have been fed, the reaction isstopped. The reactor is cooled to room temperature, the emulsion isdischarged and coagulated with HNO3 (65%). Subsequently the polymer iswashed with H₂O and dried at about 220° C.

[0082] The copolymer composition by weight is equal to: PMVE  6.1% TTD 0.72% TFE 93.18%

[0083] Polymer properties, the number of ionic end groups (measured byIR), and the extractable F⁻ value on the granule are reported in Table1.

[0084] Mechanical properties both under cold (23° C.) and under heating(250° C.) and the flex-life values are reported in Table 2.

[0085] The extractable F⁻ values obtained on extruded pipes at a lowextrusion rate equal to 0.7 and at high extrusion rate equal to 1.8meter/min are reported in Table 4.

Example 5

[0086] One works with the same autoclave of Example 1. After vacuumhaving been made, 13.9 litres of demineralized water, 1.15 g of TTD, and160 g of the above mentioned microemulsion, are in sequence introduced.The autoclave is heated up to the reaction temperature equal to 75° C.:0.35 bar of Ethane and 2.86 absolute bar of PMVE are then introduced. Agaseous mixture having the following molar ratios: TFE/PMVE=27.4;PMVE/TTD=5.83; TFE/TTD=159.83 is fed by a compressor up to a pressure of22 absolute bar.

[0087] Before the reaction starts, the gas phase results to be formed bythe following molar percentages: 81.4% TFE, 17% PMVE, 0.6% TTD, 1%Ethane. Then by a metering pump 72 cm³ of a potassium persulphate 0.0103molar solution are fed.

[0088] The polymerization pressure is maintained constant by feeding themonomeric mixture having the above defined ratios among the monomers andwhen 8,350 g of the aforesaid mixture have been fed, the reaction isstopped. The reactor is cooled to room temperature, the emulsion isdischarged and coagulated with HNO₃ (65%). Subsequently the polymer iswashed with H₂O and dried at about 220° C.

[0089] The copolymer composition by weight is equal to: PMVE  6.1% TTD 1.22% TFE 92.68%

[0090] Polymer properties, the number of ionic end groups (measured byIR), and the extractable F⁻ value on the granule are reported in Table1.

[0091] Mechanical properties both under cold (23° C.) and under heating(250° C.) and the flex-life values are reported in Table 2.

Example 6 (Comparative)

[0092] One works with the same autoclave of Example 1. After vacuumhaving been made, 13.9 litres of demineralized water, 1.5 g of TTD, and160 g of the above mentioned microemulsion, are in sequence introduced.The autoclave is heated up to the reaction temperature equal to 75° C.:0.34 bar of Ethane and 2.86 absolute bar of PMVE are then introduced. Agaseous mixture having the following molar ratios: TFE/PMVE=27.28;PMVE/TTD=3.5; TFE/TTD=95.5 is fed by a compressor up to a pressure of 22absolute bar. Before the reaction starts, the gas phase results to beformed by the following molar percentages: 81.3% TFE, 16.7% PMVE, 1.09TTD, 1% Ethane. Then by a metering pump 72 cm³ of a potassiumpersulphate 0.0103 molar solution are fed.

[0093] The polymerization pressure is maintained constant by feeding themonomeric mixture having the above defined ratios among the monomers.Contemporaneously with the feeding of 3,500 and 7,100 g of the aforesaidmixture, 36 cm³ and 18 cm³ of the potassium persulphate solution areadded by a metering pump.

[0094] The reaction is stopped when 8,800 g of the aforesaid monomericmixture have in total been fed. Then the reactor is cooled to roomtemperature, the emulsion is discharged and coagulated with HNO₃ (65%).Subsequently the polymer is washed with H₂O and dried at about 220° C.

[0095] The copolymer composition by weight is equal to: PMVE  6.1% TTD 2.0% TFE 91.9%

[0096] Polymer characteristics, the number of ionic end groups (measuredby IR), and the extractable F⁻ value on the granule are reported inTable 1.

[0097] Mechanical properties both under cold (23° C.) and under heating(250° C.) and the flex-life values are reported in Table 3.

[0098] It is noticed that as regards the mechanical properties underheating, the stress at break is lower than 5 MPa and the elongation atbreak is lower than 350%.

Example 7 (Comparative)

[0099] In the same autoclave used in Example 1, after vacuum having beenmade, 13.9 litres of demineralized water, 0.5 g of TTD, and 160 g of theabove mentioned microemulsion, are in sequence introduced.

[0100] The autoclave is heated up to the reaction temperature equal to75° C.: 0.47 absolute bar of Ethane and 1.2 absolute bar of PMVE arethen introduced. A gaseous mixture having the following molar ratios:TFE/PMVE=64.13; PMVE/TTD=4.37; TFE/TTD=280.34 is fed by a compressor upto a pressure of 22 absolute bar.

[0101] The gas phase before the reaction starts, is formed by thefollowing molar percentages: 91.2% TFE, 7.5% PMVE, 0.3% TTD, 1% Ethane.Then by a metering pump 72 cm³ of a potassium persulphate 0.0103 molarsolution are fed.

[0102] The polymerization pressure is maintained constant by feeding theaforesaid monomeric mixture and after having fed 8,800 g of theaforesaid mixture the reaction is stopped. The reactor is cooled to roomtemperature, the emulsion is discharged and coagulated with HNO₃ (65%).Subsequently the polymer is washed with H₂O and dried at about 220° C.

[0103] The copolymer composition by weight is equal to: PMVE  2.8% TTD 0.72% TFE 96.48%

[0104] Polymer characteristics, the number of ionic end groups (measuredby IR), and the extractable F⁻ value on the granule are reported inTable 1.

[0105] Mechanical properties both under cold (23° C.) and under heating(250° C.) and the flex-life values are reported in Table 3. It isnoticed that the flex life value is lower than 20,000.

Example 8 (Comparative)

[0106] One works with the autoclave of Example 1. After vacuum havingbeen made, 13.9 litres of demineralized water, 0.5 g of TTD, and 160 gof the above mentioned microemulsion, are in sequence introduced.

[0107] The autoclave is heated up to the reaction temperature equal to75° C.; 0.31 bar of Ethane and 4.34 absolute bar of PMVE are thenintroduced. A gaseous mixture having the following molar ratios:TFE/PMVE=16.64; PMVE/TTD=16.14; TFE/TTD=268.57 is fed by a compressor upto a pressure of 22 absolute bar.

[0108] The gas phase, before the reaction starts, is formed by thefollowing molar percentages: 76.33% TFE, 22.5% PMVE, 0.31% TTD, 0.86%Ethane. Then by a metering pump 72 cc of a potassium persulphate 0.0103molar solution are fed.

[0109] The polymerization pressure is maintained constant by feeding theaforesaid monomeric mixture and after 8,800 g of the aforesaid mixturehave been fed the reaction is stopped. The reactor is cooled to roomtemperature, the emulsion is discharged and coagulated with HNO₃ (65%).Subsequently the polymer is washed with H₂O and dried at about 220° C.

[0110] The copolymer composition by weight is equal to: PMVE  9.3% TTD 0.72% TFE 89.98%

[0111] Polymer characteristics, the number of ionic end groups (measuredby IR), and the extractable F⁻ value on the granule are reported inTable 1.

[0112] Mechanical properties both under cold (23° C.) and under heating(250° C.) and the flex-life values are reported in Table 3.

[0113] It is noticed that as regards the mechanical properties underheating, the stress at break is lower than 5 MPa and the elongation atbreak is lower than 350%. Besides Table 1 shows that the extractable F⁻values are higher than 1 ppm.

Example 9 (Comparative)

[0114] In the same autoclave used in Example 1, after vacuum having beenmade, 13.9 litres of demineralized water, 80 g ofperfluoropropylvinylether (PPVE) and 128 g of the above mentionedmicroemulsion, are in sequence introduced.

[0115] The autoclave is heated up to the reaction temperature equal to75° C.: 0.28 bar of Ethane and 2.86 bar of PMVE are then introduced. Agaseous mixture in a molar ratio TFE/PMVE=28.85 is fed by a compressorup to a pressure of 22 absolute bar.

[0116] The composition of the gaseous mixture present in the autoclavetop is analyzed by gaschromatography. Before the reaction starts, thegas phase results to be formed of the following molar percentages: 79.2%TFE, 16% PMVE, 4% PPVE, 0.77% Ethane. Then by a metering pump apotassium persulphate 0.0103 molar solution with a flow-rate of 170cm³/h is fed.

[0117] The polymerization pressure is maintained constant by feeding themonomeric mixture having the above defined ratios among the monomers andwhen 8.720 g of the above mixture have been fed, the reaction isstopped. The reactor is cooled to room temperature, the emulsion isdischarged and coagulated with HNO₃ (65%). Subsequently the polymer iswashed with H₂O and dried at about 220° C.

[0118] The copolymer composition by weight is equal to: PMVE  6.0% PPVE 1.0% TFE 93.0%

[0119] Polymer characteristics, the number of ionic end groups (measuredby IR), and the extractable F⁻ value on the granule are reported inTable 1. Table 1 shows that the extractable F⁻ values are higher than 1ppm.

[0120] Mechanical properties both under cold (23° C.) and under heating(250° C.) and the flex-life values are reported in Table 3.

[0121] The extractable F⁻ values on an extruded pipe at low extrusionrate (0.7 meter/minute) and at high extrusion rate (1.8 meter/minute)are reported in Table 4. TABLE 1 Composition MFI T_(II) melting Ionicend groups F⁻ 24 h Example (% by wt.) (5 Kg 372° C.) (° C.) (mol/kg)(ppm) 1  4.7% PMVE 3.0 294 2.5 × 10⁻⁴ 1.0 0.72% TTD 2  5.1% PMVE 3.5292.2 2.2 × 10⁻⁴ 0.9 0.72% TTD 3  6.1% PMVE 3.4 288.3 2.0 × 10⁻⁴ 0.90.41% TTD 4  6.1% PMVE 3.4 287 2.7 × 10⁻⁴ 0.8 0.72% TTD 5  6.1% PMVE 3.5283.5 3.0 × 10⁻⁴ 1.0 1.22% TTD 6 (Comp.)  6.1% PMVE 4 279 3.5 × 10⁻⁴ 1.0 2.0% TTD 7 (Comp.)  2.8% PMVE 2.6 305 2.5 × 10⁻⁴ 1.0 0.72% TTD 8(Comp.)  9.3% PMVE 4 265.5 6.0 × 10⁻⁴ 1.8 0.72% TTD 9 (Comp.)  6.0% PMVE3 286  10 × 10⁻⁴ 3.0  1.0% PPVE

[0122] TABLE 2 EXAMPLE 1 2 3 4 5 Copolymer composition 4.7% PMVE 5.1%PMVE 6.1% PMVE 6.1% PMVE 6.1% PMVE (% by weight) 0.72% TTD 0.72% TTD0.41% TTD 0.72% TTD 1.22% TTD Flex-life 21,000 23,000 21,000 42,00069,560 Mechanical properties at 23° C. Elastic Modulus (MPa) 487 480 437462 454 Yield stress (MPa) 14.3 14.4 14.2 14.8 15 Stress at break (MPa)28.9 29 28.5 30 32.6 Elongation at break (%) 333 335 330 305 322Mechanical properties at 250° C. Elastic Modulus (MPa) 18.8 18 16 16.813 Yield stress (MPa) 3.2 3.2 2.9 2.9 2.6 Stress at break (MPa) 5.9 5.55.1 5.6 5.5 Elongation at break (%) 405 370 359 350 380

[0123] TABLE 3 EXAMPLE 6 (Comp.) 7 (Comp.) 8 (Comp.) 9 (Comp.) Copolymercomposition 6.1% PMVE 2.8% PMVE 9.3% PMVE 6.0% PMVE (% by weight) 2.0%TTD 0.72% TTD 0.72% TTD 1.0% PPVE Flex-life 78,500 3,850 140,000 50,000Mechanical properties at 23° C. Elastic modulus (MPa) 447 474 433 481Yield stress (MPa) 15 14.0 14.2 14.5 Stress at break (MPa) 31.9 25.5 3329.8 Elongation at break (%) 310 339 382 312 Mechanical properties at250° C. Elastic modulus (MPa) 12 28 8.2 17 Yield stress (MPa) 2.4 3.81.7 2.9 Stress at break (MPa) 4.5 5.9 3.3 6.0 Elongation at break (%)320 382 310 419

[0124] TABLE 4 Pipe extrusion rate F⁻ 24 h EXAMPLE (m/min) (ppm) 4 1.80.9 0.7 0.9 9 (Comp.) 1.8 2.1 0.7 2.0

1. A thermoprocessable copolymer of TFE consisting essentially of: (a)from 4.5 to 8.5w by weight of perfluoromethylvinylether (PMVE); (b) from0.3 to 1.6% by weight of a perfluorodioxole of formula:

 wherein: Y=F, OR_(f), R_(f) wherein R_(f) is a perfluoroalkyl havingfrom 1 to 5 carbon atoms;  X₁ and X₂, equal to or diferent from eachother, are —F or —CF₃; (c) TFE, forming the remaining part to 100%; saidcopolymer having: a number of ionic end groups lower than 4×10⁻⁴ mol/Kgof polymer; flex life values higher than 20,000 in a MFI range (measuredat 372° C. with a 5 Kg load) comprised between 1 and 5; release valuesof F⁻ ions on the polymer in pellet and on the finished manufacturedarticle lower than 1 ppm by weight with respect to the polymer weight;release values of F⁻ ions lower than 1 ppm on extruded manufacturedarticles, both at low and high extrusion rate of the polymer.
 2. Athermoprocessable TFE copolymer according to claim 1, wherein themonomer (b) has Y=OR_(f) where R_(f)=—CF₃; X₁,X₂=F.
 3. A process forobtaining the copolymers according to claims 1-2, wherein thepolymerization in aqueous medium is carried out in emulsion or inmicroemulsion at a temperature in the range 25°-120° C., preferably 60°C.-95° C., at a pressure in the range 10-50 absolute bar, in thepresence of: an inorganic initiator of radical type, in such an amountas to produce a number of ionic end groups lower than 4×10⁻⁴ mol/Kg ofpolymer: a chain transfer agent selected among hydrogen, methane,ethane, propane.
 4. A process according to claim 3, wherein theinorganic initiator is an alkaline metal or ammonium persulphate.
 5. Useof the copolymers according to claims 1-2, for obtaining pipes byextrusion.
 6. Use of the copolymers according to claims 1-2, forobtaining fittings, connections for pipings, by moulding.